Memory technologies employ dynamic random access memory (DRAM) and ferroelectric random access memory (FRAM) cells. These cells employ a large number of devices that are capable of holding a charge, such as capacitors. Generally, a capacitor has a set of electrodes separated by an insulator. Typically, many capacitors are required to provide useful quantities of storage capability. The amount of charge that can be stored on a capacitor is termed its capacitance. Capacitance is affected by the area of the conductive surfaces (i.e., the electrodes), the separation distance of the conductive surfaces, and the material separating the conductive surfaces (i.e., the insulator or dielectric). As DRAM or FRAM cell size restrictions become more stringent (i.e., smaller) because of the need for higher-density storage capability, the capacitance requirements for an individual capacitor are expected to remain the same. Therefore there is a need for RAM structures and materials capable of meeting the capacitance requirements while allowing for reduced cell size.
For a parallel plate capacitor with a dielectric, the capacitance is given by:
  C  =      k    ⁢                                        a            o                    o                ⁢        A            d      Wherein,    C=capacitance,    k=dielectric constant    d=plate separation (space enclosed),    A=plate area, and    a00=permitivity of free space.
According to the equation, capacitance is proportional to the plate area and dielectric constant, but inversely proportional to the capacitor electrode spacing. These three variables have an effect on the work function and the leakage current, which are used to evaluate capacitor performance. Work function is the minimum level of energy needed to remove an electron from the Fermi level of a metal to infinity. McGraw-Hill Dictionary of Scientific and Technical Terms, 5th ed. The work function and leakage current requirements must be adhered to when the spacing, dielectric constant, or area are changed.
The dielectric constant k is a property of any material. Traditionally, SiO2 and Si3N4 materials have been used as dielectrics in microelectronic workpiece processing, but future generations of DRAM necessitate the use of high-k dielectric materials. High-k dielectric materials are presently being investigated for integration into DRAM devices.
High-k materials have an associated electron affinity greater than low-k materials. Electron affinity is defined to mean the work needed in removing an electron from a negative ion, thus restoring the neutrality of an atom or molecule. In theory, if the electron affinity of a dielectric is close to the work function of an electrode, it is easy for electrons to move from the metal into the non-metal. Electrode materials must be chosen so that the electrode work function is greater than the electron affinity of the dielectric otherwise, electrons migrate to the conduction band producing a net transport of charge, meaning current will bleed from the capacitor. This occurrence is termed leakage current. Therefore, future high-k dielectrics will require electrodes with large work functions.
Typically, platinum has been considered a suitable choice for capacitor electrodes because its inertness, resistance to oxygen diffusion, and high work function leads to low leakage current and a high breakdown voltage. Platinum is known to have a work function of 5.6 to 5.7 (eV). (Ba, Sr) TiO3 dielectrics for future stacked-capacitor DRAM, IBM Journal of Research and Development, Vol. 43, No. 3, Kotecki et al. Electrochemically deposited platinum provides numerous advantages, for example, high conformality, a high deposition rate, and minimal platinum etching on patterned surfaces. Suitable methods and apparatus for electrochemically depositing platinum on a surface of a microelectronic workpiece have been described by the present inventors in U.S. patent application Ser. No. 09/429,446.
Despite the suitability of platinum as a material for capacitor electrodes, future generation capacitor designs employing high-K dielectric materials will require that electrodes be made from materials that exhibit a suitable resistivity, a work function even higher than platinum, and permeability to oxygen that is less than platinum.
It is believed that platinum alloys would be a suitable electrode material and exhibit a higher work function and a lower permeability to oxygen than platinum. Therefore, the need exists to develop plating compositions and methods of forming noble metal alloy features on the surface of a microelectronic workpiece.